I'd listen to the the two posters above.
A separate stereo amp is going to do probably absolutely nothing to improve the "sound quality" of your music, unless the Onkyo is having trouble driving the speakers, which I very, very much doubt (how large is your room, BTW?)
It's more likely that either the recording you are listening to is badly recorded, or that you are listening to highly compressed music which has audible compression artifacts, or you are simply sitting below the twitter level -- I don't know why B&Ws are so popular, but many of their models have pretty mediocre off-center coverage and going off center will cause a dip.
Moreover, many B&Ws seem to have large dips right in the 1-4kHz band, which is right where the human ear is most sensitive -- a 2dB dip at 2kHz is likely to be audible, while a 20dB dip at 80Hz is likely not.
Anyway, the short answer is, don't bother with a stereo amp, but rather invest the $$$ either in a sub (you do not mention if you have one) or in better speakers.
P.S. Just saw your post about the music you listen to. It's worth noting that while Spotify advertises 320k, it appears that the majority (they refuse to disclose what portion) of their streams are still at 160k, which would make a noticeable difference. That's why I use MOG, which streams mostly at 320k.